The present invention relates to a combustion-powered nail gun that generates drive force by igniting a fuel/air mixture to drive a fastener such as a nail into a work piece.
U.S. Pat. No. 4,403,722, 4,483,280 (U.S. Re. Pat. No. 32,452), U.S. Pat. Nos. 4,483,473, and 4,483,474 disclose combustion-powered tool assemblies. FIG. 8 schematically shows configuration of a conventional combustion-powered nail gun 100 similar to that disclosed in these U.S. Patents. The nail gun 100 includes a housing 114 to which a handle 111, a tail cover 117, a push lever 121, and a magazine 113 are disposed.
The housing 114 accommodates therein a head cover 123, a combustion chamber frame 115, a cylinder 104, and a piston 110. The combustion chamber frame 115, the head cover 123, and the piston 110 together define a combustion chamber 105. Further, the piston 110 divides the internal space of the cylinder 104 and the combustion chamber frame 115 into upper chamber S2 inclusive of the combustion chamber 105 and a lower chamber S1. The head cover 123 and the cylinder 104 are fixed to the housing 114. The combustion chamber frame 115 is vertically movable within the housing 114 as guided by the housing 114 and the cylinder 104. The upper end of the combustion chamber 115 can be seated on the head cover 123 to provide the sealed combustion chamber 105. Although not shown in the drawings, a connection rod linkingly connects the combustion chamber frame 115 with the push lever 121 so that the combustion chamber frame 115 and the push lever 121 move together in an interlocking relation to each other.
Further, a spring (not shown) is provided for urging the push lever 121 downward. Therefore, the push lever 121 and the combustion chamber frame 115 are urged downwardly while no force operates against the urging force of the spring. At this time, because the head cover 123 and the cylinder 104 are fixed, an inlet (not shown) is opened between the head cover 123 and a top end of the combustion chamber frame 115, and an outlet (not shown) is opened between the upper outer peripheral portion of the cylinder 104 and the combustion chamber frame 115. Although not shown in the drawings, annular seals for forming tight seals at the inlet and the outlet are provided at the lower end of the head cover 123 and the upper end of the cylinder 104. Further, an intake vent (not shown) is provided in the upper end of the housing 114, and a discharge vent (not shown) is provided in the lower end of the housing 114.
The housing 114 further accommodates a motor (not shown), a spark plug 109 in a space above the head cover 123. Further, a fuel canister 107 holding a fuel is disposed in the housing 114. An injection port (not shown) connects the fuel canister 107 for supplying combustible gas from the fuel canister 107 into the combustion chamber 105. A fan 106 is disposed in the combustion chamber 105. The fan 106 is attached to and rotated by the drive shaft of the motor (not shown). Electrodes of the spark plug 109 are exposed to the combustion chamber 105. Ribs 124 are provided on the inner surface of the combustion chamber frame 115 so as to protrude radially inwardly of the combustion chamber 105.
A seal ring (not shown) is held at an outer peripheral surface of the piston 110 so as to be slidably movable with respect to the cylinder 104. A bumper (not shown) is provided in the cylinder 104 and below the piston 110 for absorbing excessive energy of the piston 110 after a nail driving operation. Also, an exhaust hole (not shown) is formed in the cylinder 104. A check valve (not shown) of well-known construction is provided on the outer side of the exhaust hole. A driver blade 116 extends from the piston 110 toward the tail cover 117 for driving a nail. A trigger spring 112A is connected the trigger switch 112 for biasing the trigger switch 112 toward its OFF position.
The handle 111 is attached to a middle section of the housing 114. A trigger switch 112 is provided on the handle 111. The trigger switch 112 is biased by a trigger switch spring 112A for urging the trigger switch 112 toward its OFF position. Each time the trigger switch 112 is pulled (turned ON), the spark plug 109 generates a spark if the sealed combustion chamber 105 is provided.
The magazine 113 and the tail cover 117 are attached to the lower end of the housing 114. The magazine 113 is filled with nails (not shown). The magazine 113 feeds the nails one at a time to the tail cover 117. The tail cover 117 sets the nails fed from the magazine 113 in a position below the driver blade 116 and guides movement of the nails when the nails are driven downward by the driver blade 116 into a workpiece W.
A mechanism 200 for maintaining closing state of the combustion chamber 105 is provided. The mechanism 200 includes a trigger switch bracket 201 extending from the trigger switch 112, a rod 202 extending from the combustion chamber frame 115, and a cam member 203. The trigger switch bracket 201 has a lower end provided with a pivot pin 205. The cam member 203 has a slot opening 206 engaged with the pivot pin 205. The cam 203 is pivotally connected to the housing 114 by a pivot bush 207, and has a first stop surface 208 selectively engageable with a lower end of the rod 202. Further, the cam 203 has a second stop surface 209 for preventing manipulation of the trigger switch 112.
When the combustion chamber frame 115 is separated from the head cover 123 by the biasing force of the spring, the rod 202 is positioned beside the second stop surface 209, so that counterclockwise pivotal movement of the cam 203 is prevented, thereby preventing upward movement of the trigger switch 112. When the combustion chamber frame 115 is seated onto the head cover 123, the rod 202 is moved away from the second stop surface 209, so as to allow counterclockwise movement of the cam 203. In this state, if the trigger switch 112 is pulled upwardly (turned ON) against the biasing force of the trigger switch spring 112A, the cam 203 is pivotally moved in the counterclockwise direction, so that the lower end of the rod 202 can be seated on the first stop surface 208. As a result, downward movement of the combustion chamber frame 115 is prevented by the abutment between the rod 202 and the first stop surface 208.
If the tool 100 is moved away from the workpiece W and if the trigger switch 112 is released, the cam 203 can be piviotally moved in a clockwise direction by the biasing force of the trigger switch spring 112A, so that the lower end of the rod 202 slides over the first stop surface 208, and can be positioned beside the second stop surface 209.
In the conventional combustion-powered nail gun, the piston 110 is moved to its lower dead center as a result of combustion, and the piston 110 is returned to its original upper dead center by the pressure difference between the upper chamber S2 and the lower chamber S1. After the combustion, negative pressure is generated in the upper chamber S2 because high pressure combustion gas is discharged through the exhaust hole and the check valve and because heat of the combustion chamber 105 is gradually absorbed into the cylinder 104 and the combustion chamber frame 115 to lower the internal pressure. This is generally referred to as “thermal vacuum”. On the other hand, atmospheric pressure is applied in the lower chamber S1. Thus, the piston 110 can be moved toward its upper dead center.
However, the internal pressure of upper chamber S2 is increased in accordance with the movement of piston 110 toward its upper dead center, if the cooling speed cannot provide the pressure decrease in the upper chamber S2. Accordingly, the moving speed of the piston 110 toward its upper dead center is lowered or greatly varied dependent on cooling speed (pressure reducing speed) of the upper chamber S2. Consequently, one shot cycle requires a prolonged period, and an operator may be fatigued from such driving work. This is particularly disadvantageous in case of a repeating shot type nail gun in which a trigger switch 112 is maintained in its ON position while successively driving a plurality of nails at different locations of the workpiece W by repeatedly pushing and releasing the push lever 121 toward and away from the workpiece W.